1. Field of the Invention
The present invention relates to a method for manufacturing a thin film transistor and a method for manufacturing a display device including the thin film transistor at least in a pixel portion.
2. Description of the Related Art
In recent years, techniques to form thin film transistors using, for a channel formation region, a semiconductor thin film (with a thickness of approximately several tens to several hundreds of nanometers) which is formed over a substrate having an insulating surface have attracted attention. The thin film transistor is widely applied to electronic devices such as ICs and an electro-optical device. Rapid development of a thin film transistor particularly as a switching element of an image display device is required.
As a switching element in an image display device, a thin film transistor in which an amorphous semiconductor film is used for a channel formation region, a thin film transistor in which a polycrystalline semiconductor film is used for a channel formation region, or the like is used. As a method of forming a polycrystalline semiconductor film, there is known a technique in which a pulsed excimer laser beam is shaped into a linear laser beam by an optical system and an amorphous silicon film is crystallized by being irradiated while being scanned with the linear laser beam.
As a switching element in an image display device, further, a thin film transistor in which a microcrystalline semiconductor film is used for a channel formation region is used (see Reference 1: Japanese Published Patent Application No. H4-242724; and Reference 2 : Japanese Published Patent Application No. 2005-49832).
A thin film transistor in which a polycrystalline semiconductor film is used for a channel formation region has an advantage that the mobility is higher than that of a thin film transistor in which an amorphous semiconductor film is used for a channel formation region by two or more digits, and a pixel portion and a peripheral driver circuit of a semiconductor display device can be formed over the same substrate. However, the thin film transistor in which a polycrystalline semiconductor film is used for a channel formation region requires a more complicated process than the thin film transistor in which an amorphous semiconductor film is used for a channel formation region because of crystallization of the semiconductor film. Thus, there are problems such as a reduction in yield and an increase in cost.
Further, an inversely-staggered thin film transistor in which a microcrystalline semiconductor film is used for a channel formation region has problems in that the crystallinity of an interface region between a gate insulating film and the microcrystalline semiconductor film is low and electric characteristics of the thin film transistor are poor.
In view of the above problems, it is an object of the present invention to provide a method for manufacturing a thin film transistor with excellent electric characteristics and high reliability and a display device including the thin film transistor with high yield.